1. Field of the Invention
The present invention is concerned with electrical power conversion by means of three-phase transformers in electrical networks. It relates to a three-phase transformer of the kind described in the preamble of the first claim.
2. Discussion of Background
In mesh-connected electrical power supply networks or ring networks, on many occasions it is necessary to change the current distribution in the network in order to protect a path against overloading or in order to achieve maximum efficiency in the overall current distribution, in other words minimum network losses. This becomes possible by inserting a voltage into the ring network, which voltage effects circulating currents. The result of superposition of said circulating currents on the previous network currents is a changed current distribution in the network and hence also in the transformers which are installed in the network and are operated in parallel.
Adding an additional voltage U.sub.Z in a network path can influence both the active volt-amperes distribution and the reactive volt-amperes distribution in a desirable manner within the network. For this purpose, the additional voltage U.sub.Z has a so-called in-phase component, which is in phase with the power supply voltage, and a quadrature component, which is phase-shifted by 90.degree. with respect to the power supply voltage. A change in the in-phase component influences the reactive volt-amperes distribution in the network and the variation of the quadrature component controls the active volt-amperes flow. A combination of in-phase and quadrature regulation is called phase-angle regulation. The so-called phase-angle regulation consequently influences both the reactive volt-amperes distribution and the active volt-amperes distribution in the network.
In three-phase power supply networks, an additional voltage U.sub.Z acting according to the above principle is usually realized by means of three-phase autotransformers, as is described for example in "Elektrische Maschinen" [Electric Machines] by Bodefeld/Sequenz, Springer Verlag 1971, on pages 86 ff. A normal star-type autotransformer having various voltage taps is used to control the in-phase component, and is called in-phase regulating transformer in the text below. The control of the quadrature component is mainly performed by a delta-type autotransformer (likewise designed as a series transformer), a so-called quadrature regulating transformer. Accordingly, for combined in-phase and quadrature regulation in a network path, a main transformer is connected in series with an in-phase regulating transformer and a quadrature regulating transformer. The main transformer serves for the coupling of the high-voltage network and low-voltage network, and the series-connected in-phase and quadrature regulating transformers permit, by regulation, lower-loss and hence more economical utilization of transformers connected in parallel. However, the series circuit described has the disadvantage of the high production costs for the additional regulating transformers and their extensive space requirement. Furthermore, in-phase and quadrature regulating transformers are likewise lossy and exhibit, specifically, copper and core losses.